1 Field of the Invention
The invention relates to dispersing gas into molten metal and, more particularly, to techniques for causing finely divided gas bubbles to be dispersed uniformly throughout the molten metal.
2. Description of the Prior Art
In the course of processing molten metals, it sometimes is necessary to treat the metals with gas. For example, it is customary to introduce gases such as nitrogen and argon into molten aluminum and molten aluminum alloys in order to remove undesirable constituents such as hydrogen gas, non-metallic inclusions, and alkali metals. The gases added to the molten metal chemically react with the undesired constituents to convert them to a form (such as a precipitate or a dross) that can be separated readily from the remainder of the molten metal. In order to obtain the best possible results, it is necessary that the gas be combined with the undesirable constituents efficiently. Such a result requires that the gas be dispersed in bubbles as small as possible and that the bubbles be distributed uniformly throughout the molten metal.
As used herein, reference to "molten metal" will be understood to mean any metal such as aluminum, copper, iron, and alloys thereof, which are amenable to gas purification. Further, the term "gas" will be understood to mean any gas or combination of gases, including argon, nitrogen, chlorine, freon, and the like, that have a purifying effect upon molten metals with which they are mixed.
Heretofore, gases have been mixed with molten metals by injection through stationary members such as lances, or through porous diffusers. Such techniques suffer from the drawback that inadequate dispersion of the gas throughout the molten metal can occur. In order to improve the dispersion of the gas throughout the molten metal, it is known to stir the molten metal or otherwise convey it past the source of gas injection. Devices also are known that accomplish both of these functions, that is, the devices stir the molten metal while simultaneously injecting gas into the molten metal.
Despite the existence of combined stirring/injecting device, certain problems remain. Combined stirring/injecting devices often exhibit poor stirring action. Sometimes cavitation occurs or a vortex is established that moves around the inside of the vessel within which the molten metal is contained. Frequently these devices dispense bubbles that are too large or which are not uniformly distributed throughout the molten metal. A problem with one known prior device is that it utilizes an impeller having passageways that can be clogged with dross or foreign objects. Most of the prior devices are expensive, complex, and usable with only one type of molten metal-handling system. Other problems frequently encountered are poor longevity of the devices due to oxidation, erosion, or lack of mechanical strength. These latter concerns are particularly troublesome in the case of aluminum because the stirring/injecting devices usually are made of graphite, and graphite is rapidly oxidized and eroded by molten aluminum. Accordingly, devices that initially perform adequately often become quickly oxidized and eroded so that their mixing and gas dispersing effectiveness diminishes rapidly; in severe cases, complete mechanical failure can occur.